A commenter points to the long-awaited release of a preprint from the XENON100 experiment giving results from a 100-day run last year. This is the most sensitive dark matter experiment that has released data. The result: with an expected background of 1.8 +/- .6 events, they see 3 events (i.e. about what you’d expect if there’s nothing there). For a WIMP mass of 50 GeV, this allows them to exclude certain WIMP cross-sections at the level of 7.0 x 10-45cm2. This pretty conclusively kills off some other claims by dark matter experiments to have seen something, especially the CDMS result from late 2009 (see here).

One motivation for supersymmetry has always been that it can provide a WIMP with the right properties to explain astrophysical dark matter observations. This new data rules out some (if you use the SUSY expectations plotted in the new paper), or most (if you use the expectations plotted in the CDMS paper, see here and here) of the possible parameter space where such a particle is expected, providing yet another nail in the SUSY coffin.

Update: More details available at Resonaances and Tommaso Dorigo’s blog.

Update: For a detailed analysis of the implications of the XENON100 result for supersymmetry models, see here.

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106 Responses to No WIMPs

  1. Kea says:

    There may be many nails in Susy’s coffin, but Susy is still running around, terrorising people as a zombie.

  2. Susy WIMP says:

    The New York Times reports the finding, with some collaborators posing for a photo:
    Particle Hunt Nets Almost Nothing; the Hunters Are Almost Thrilled

  3. Georges says:

    This is excellent news for all those theories that predict the absence of supersymmetry. That is only a tiny fraction of all candidate theories, but it is an important step that tells us where to continue searching. What a great day.

    And compliments for being faster than Resonaances on this news item!

    Peter, is there an overview of candidate theories that do not contain supersymmetry somewhere?

  4. Holycow says:

    Your take on this is really a disgrace. The result is not bad for SUSY and the most important point is that no WIMP of any kind is there (yet), SUSY or non-SUSY. To intrepret this just as “another nail in the SUSY coffin” is only another instance of your legendary theoretical shortsightedness. Paraphrasing the classics, “the dead that you kill, are in excellent health”.

  5. Peter Woit says:


    What’s a disgrace is the refusal of SUSY proponents to admit that there is such a thing as experimental evidence against their pet idea. Imagine that XENON100 had seen a signal. You can be sure that SUSY advocates would be loudly proclaiming that this was exactly what they expected, that evidence for SUSY had been found.

    More specifically, the John Conway and Tommaso Dorigo 2008 postings that I linked to argued that the data then was already starting to cut deeply into the region favored by SUSY. Now that the limits have been moved down more than an order of magnitude, wiping out the “favored region” on the plot they show, why is this not evidence against SUSY?

  6. DB says:


    Susy is like Count Dracula. It will take more than a few nails in the coffin to put paid to him. He already has a few exit routes mapped out. But he is definitely on the run.

    I expect Tommaso Dorigo’s credit rating to be upgraded by Moody’s any day now.

  7. Georges says:


    you come close to say that nothing can be bad for SUSY, which would mean that SUSY is not science. But I am sure you do not want to say that.

    So please tell us what else would be bad for SUSY – we are all listening.

  8. Rien says:

    Actually, CDMS did not claim to have seen anything. They had two candidate events, but this was not significant.

  9. Peter Woit says:


    Many people did at the time interpret the CDMS two events as “seeing something”, although not conclusively. One example from the many news stories at the time:


    “Even so, Joseph Lykken, a theorist at Fermilab, says he’s relieved that CDMS has finally seen something. WIMPs are predicted to exist by theories involving a principle called supersymmetry, which posits a heavy partner for every particle currently known. Had CDMS continued to see nothing, the results would have undermined those theories. So seeing something is better than seeing nothing, Lykken says.”

  10. Holycow says:

    Clarification. What I find annoying in this post is that Peter seems to be so happy about the loss of another chunk of SUSY parameter space that forgets being worried by the absence of a positiva signal, of any kind (Or do you believe that WIMPs and SUSY are equivalent?). And that’s the most important fact. You should get your priorities right.

    Then, SUSY can be disproved by LHC with more luminosity
    (certainly not with 35/pb).

  11. Rien says:

    Peter: yes, this is true, but it was over-eager theorists, not the experiment themselves that thought they were real. They even went back to check those two candidates again, IIRC, and concluded they were background.

  12. Paul Wells says:

    Dear Holycow,

    What Luninosity at what energy would be sufficient to disprove SUSY ? 1 fb at 7 Tev ?


  13. Peter Woit says:


    I’m happy that the XENON100 experiment is a success, accomplishing what it is supposed to do. That they get a null result tells us something new and non-trivial, which is great. A positive result would have been Nobel prize-worthy and revolutionized the field, but it’s hard to call a null result very disappointing, since it’s the one that seemed far and away the most likely.

    That the null result makes one of the main arguments for SUSY less compelling is just a fact that I’m reporting.

  14. Holycow says:

    Peter, can you also report on how Xenon results affect the very possible and relevant SUSY idea of gravitinos as DM? Or explain to other readers how it is enough to exclude some chunck of parameter space to be ready to give up on the best theoretical idea we’ve had in the last 30 years?

    Paul, say 30/fb at 7TeV should be quite good. Of course for you it would be much simpler: as soon as SUSY particles start to pop up you can join the celebration (and no problem with that).

  15. chris says:

    “What I find annoying in this post is that Peter seems to be so happy about the loss of another chunk of SUSY parameter space”

    i for once am very happy. but that might be due to the fact that i am in the business of cutting parameter spaces for SM extensions myself.

    the best thing you can get is evidence for new physics. the second best thing is a good weeding of obsolete theories.

  16. Eric says:

    I think what needs to be pointed out here is that SUSY can still be observed at low energies without the dark matter actually being a SUSY WIMP. First, the LSP is only stable if R-parity is conserved, which doesn’t necessarily have to be so. Also, even if R-parity is conserved, the resulting relic density can be vanishingly small, or the LSP could be a gravitino. So, these direct detect experiments only constrain the hypothesis that dark matter is a SUSY WIMP. Superpartners could still be observed at the LHC without also needing to explain the dark matter.

  17. Peter Woit says:


    OK, here’s a report on what XENON100 has to say about the idea that, since the neutralino idea didn’t work, let’s save SUSY by postulating the gravitino as DM:

    Nada, Zip, Nothing. This is an idea you can’t test with this kind of experiment.

    As for your description of supersymmetric extensions of the SM as “the best theoretical idea we’ve had in the last 30 years”, I’m afraid I think it’s more accurate to describe it as “a not very good idea that has done huge damage to the field for 30 years by dominating the attention of theorists, for no good reason”.

  18. Holycow says:

    Dear Peter,

    if you have proof that “the neutralino idea doesn’t work” you better publish it as many people would be interested in knowing. If the lack of a signal in some region of parameter space is enough for you to give up on some idea, then this just goes to show you are a bad scientist (bad theorist and would also be a bad experimentalist) and that you let your prejudices rule over logic.

    You write a blog that journalists read and you should be more careful with the way you project your prejudices (and personal frustrations) on your reports.

    “Since the neutralino idea didn’t work, let’s save SUSY by postulating the gravitino as DM”. Your naive simplistic point of view is wrong again. This idea has been circulating for years and has its own appeal. The point you fail to appreciate is that it is damn difficult to guess what the theory of nature is without experimental guidance. By your comments you also fail in discriminating the sensible theoretical ideas.

    “Nada, Zip, Nothing. This is an idea you can’t test with this kind of experiment.” Correct, so your claim about coffins and nails is pure hot air.

    “a not very good idea that has done huge damage to the field for 30 years by dominating the attention of theorists, for no good reason”. Which as Lubos would say just goes to show your limitations as a theoretical physicist.

  19. Peter Woit says:


    I happen to think that the scientific accuracy of what I write here is quite a bit higher that that of what well-known theorists promoting SUSY/extra dimensions/string theory have to say to the press. To the extent journalists do pay any attention to this blog, that’s one reason.

    If a low opinion of SUSY extensions of the SM makes one a bad theoretical physicist, yes, I’m a bad theoretical physicist.

  20. Georges says:


    SUSY is by far not “best theoretical idea we’ve had in the last 30 years”. I do not like string theory too much, but it is clear that strings themselves are a much better idea than supersymmetry. Strings were much more successful than supersymmetry by any measure you might want to use. And strings are not the best idea either, as Peter consistently showed.

    Fact is that supersymmetry is wrong. Go back to the drawing board. And better be quick – better ideas, which agree with data, are already on the market.

  21. Anon says:


    I highly advise anyone who is actually trying to learn about physics by reading this blog, to look briefly into the differences between gauge mediated and gravity mediated supersymmetry breaking. That will teach you a lot more about what susy does or does not predict, than reading oversimplified statements here will.

    These distinctions became clear in the early 80s and are not a response to any recent experiment, as a brief perusal of the many thousands of papers will teach you.

  22. Holycow says:

    Georges, agreed, string theory is even better. I was just thinking of the TeV scale and the LHC.

    “Fact is that supersymmetry is wrong.”
    Fact probably has a different meaning for you than for the rest of scientists.
    Fact is, it’s pretty hard work to come up with good theoretical ideas and even harder to find new stuff experimentally. You should show a bit more respect for this enterprise and present a more serious perspective of what this is all about. It’s OK if you have your personal preferences for whatever you expect to show up at the LHC but don’t misrepresent the meaning of experimental results for the theories you dislike. Be honest. I think that’s a minimum to ask.

    “better ideas, which agree with data, are already on the market.” Really? Like what?

  23. Mike says:


    As a laymen, I have some questions. Could you please say (simply please) what SUSY does predict? Then, could you please explain what experimental results would falsify such predictions? Finally, could you please state whether, in your opinion, current experimental set-ups, if any, are capable of falsifying such predictions? Thanks.

  24. Peter Woit says:

    To be clear, I don’t agree with Georges that good ideas about how to get beyond the SM that agree with data exist.

    Mike asks an excellent question, and I’m curious to hear what SUSY advocates have to say about this now, as well as to look at the record and see what they were saying earlier. So far the reaction I’ve seen to null results from the LHC and dark matter experiments has been “that’s no problem at all for SUSY, although if it had come out the other way, it would have been strong evidence for SUSY”, which strikes me as less than honest. Or, if SUSY is going to be pursued scientifically by the new standards developed to deal with the failure of string theory (experiment can never provide evidence against the idea, only evidence for it), that should be made explicit.

  25. Holycow says:

    So, even Peter should be able to understand this allegory: you go pick up your friends who went for a long walk in the forest and in your way to meet them you lose your car keys. You discover this when you find them, explain the situation and then you start your search. As time passes and no keys are found, your friend Peter starts ranting that there were no keys at all and that the null result of your search just proves it. Well, you know that this simply means you haven’t covered the whole path. But then somebody comes along with a brand new metal detector and you continue your search much faster and happier. You cover 1/4 of the whole path you walked in no time and here comes Peter repeating again and again there are no keys because the whole idea is preposterous and there are no such things as keys in the world. The best you could do is to ignore him and patiently continue your search, till you find the keys, or some gold coin or nothing, because somebody took them and you’ll be faced with a different problem to solve. As you see the analogy is not perfect, but really captures well the role of the chorus of naysayers in this business: Just a bloody pain in the ass.

  26. Mike says:

    I was asking a serious question, and perhaps my lack of grounding in the subject is why I don’t follow your “allegory.”

    So, you lost your keys, you know already that you lost your keys –they’re just misplaced.

    However, unless I’m missing something, even those who think SUSY exists (I’m agnostic on this — I simply don’t know), don’t know for sure that it exists — they haven’t “lost” it anywhere. What’s the connection?

    Perhaps you could help me understand. What SUSY does predict? What experimental results would falsify such predictions? In your opinion, are current experimental set-ups, if any, capable of falsifying such predictions? If not, what more is needed, other than time to keep searching? Thanks.

  27. Georges says:

    Peter, Holycow,

    there are several ideas on the market that state that there is simply *nothing* beyond the standard model plus Higgs. Such ideas agree with all data. These ideas then have to explain the shortcomings of the standard model in a way or another; but when they do, they have fewer assumptions than all those ideas that use supersymmetry or higher dimensions.

  28. Math Student says:


    A better analogy could be something like SETI. Everybody would agree that extraterrestrial intelligent life (or SUSY) exists with very little positive data but falsyfying it could require close to an infinite amount of data, and is thus impossible.

  29. Geometrick says:

    As a math PhD student myself, the previous commenter is way off the mark. The analogy they used is actually a horrendous analogy. It needs to be pointed out.

  30. Holycow says:

    Sorry, my analogy was devised to explain the chorus, not the search.

  31. Holycow says:

    Mike: one basic prediction of SUSY is that for every type of particle we know there is another with the same quantum numbers but different statistics (boson fermion) and mass. So LHC should find strongly interacting partners of the known quarks and gluons, among other things.

    SUSY theories do more than this and help solve some of the problems of the Standard Model. For such solutions to be natural the mass scale of these new particles cannot be much higher than the TeV scale, which is the range that will get covered by LHC. If the limits on such masses are pushed well in the few TeV
    regime then one should give up on this idea. But talking about the demise of SUSY now, is plain stupid.

  32. Mike says:

    Math Student,

    Thanks, that does seem like a more apt analogy.

    I’m agnostic on that question as well — but I admit to a bias that extraterrestrial life should exist, all based on my limited understanding of the nature of the physics and the scope of space and time.

    One possible difference I see is that the answer to the ET question is either yes or no — in the sense that there is no other possibility. And, although the answer to the SUSY question is also yes or no –there may well be other competing possibilities (at least in theory) for answering the questions at hand regarding what, if anything, extends beyond the SM, and any such theory has some realistic hope of being falsified in some reasonable period of time.

    On the other hand, perhaps that’s a distinction without a difference — and I’m just moving the goal post in the middle of the game.

    If so, rather than drop what I see to be an important aspect in the progress of science and the growth of knowledge — at least some hope of falsifiability — I suppose I would in the end have to say that SETI is a great and useful “hobby” for humans to undertake, but perhaps not “science” as such.

    Thanks again.

  33. Mike says:


    Thanks for your comment as well.

    I take it then, within current experimental design limits, if the LHC fails to observe strongly interacting partners of the known quarks and gluons, among other things, and absent any other new discoveries that would put it back in play, at least in your mind SUSY would be effectively falsified. Is that correct?

    Thanks again for your comment. I appreciate people trying to deal seriously with my necessarily naive questions.

  34. Peter Woit says:


    As others pointed out, your extended analogy was rather bizarre. You have never seen the keys in question.

    I take your answer to Mike to be that you’ll give up on SUSY once limits on superpartners are “well in the few TeV” regime. Hopefully the ultimate reach of the LHC will be sufficient to satisfy you. Personally I don’t see why you have to go all the way up into the few TeV regime to falsify the idea that supersymmetry explains the stabilization of the weak scale at around 100 GeV.

    I don’t expect SUSY proponents to be conclusively giving up now, but they might want to admit that these experimental results are not encouraging. For a long time, SUSY phenomenologists have been playing the game of producing elaborate plots showing what are supposedly the most likely ranges of SUSY parameters. As experiments start to rule out large sections of these ranges, the reaction seems to be to not admit that one’s predictions are starting to fail, but to produce new predictions.

  35. Tommaso says:

    Hi all,

    indeed, in 2008 the searches were over one order of magnitude less sensitive to the presence of SUSY wimps. Still, they were already clipping out regions of phase space for SUSY theories. What was true back then, is even more so now. A neutralino may be there -and I would be quite happy if we found it at the LHC, or even if one of these sit-and-wait experiments found it- but we are Bayesians by nature, at least somewhere close to our amygdala. And if the prior belief in SUSY was what it was four years ago, we have to acknowledge that it has shrunk quite sizably now.

    As for my bet, those 1000$ will be invested in a party in a suitable place. The losers of the bet (Watts and Distler) will of course be invited. Reservations are open, RSVP.


  36. The Cosmist says:

    If you don’t mind another layman’s (meta-)question, mine is this: how would you characterize the state of theoretical physics today? From the outside it sounds like there is huge disagreement and confusion about basic questions, as if the whole field is in disarray like never before. It’s strange to hear physicists arguing about experimental results and interpreting them so differently. WTF is going on with physics these days?

  37. Mike says:

    The Cosmist,

    “WTF is going on with physics these days?”

    A layman’s answer to a layman’s question. Being an optimist and assuming, as I do, that new fundamental scientific explanations are almost always surprising in many ways, I would judge the current disorder as a good sign — for surely it is now just a matter of time (short I would hope) until a better fundamental explanation emerges.

    Of course, this will only give rise to new and better questions, but that’s the name of the game.

  38. Giotis says:

    There is a clear distinction between SUSY as a fundamental property of nature and its breaking scale. Low energy SUSY which LHC tries to find is just a convenient estimation of this breaking scale engineered to help people solve some SM problems; nothing more. It may well be that these problems may not be solved by Low SUSY after all but that doesn’t mean SUSY doesn’t exist as a fundamental property of nature. There are models where SUSY’s breaking scale reaches the string scale. So SUSY cannot be falsified by LHC or any similar experiments.

  39. Mike says:


    Question: Do the models where SUSY’s breaking scale reaches the string scale encompass and purport to solve the SM problems?

  40. Giotis says:

    Yes in some cases but by other means. These models are constructed by String theorists not particle/field theorists and String theorists have other alternatives (not just Low SUSY) to solve these problems. LoW SUSY is the most obvious way but not the only one.

  41. Mike says:


    Thanks. I assume that experiments at the string scale are far beyond our current and reasonably foreseeable capability. If that’s the case, maybe Math Student’s analogy to SETI is more apt than I imagined.

  42. Peter Woit says:

    The Cosmist,

    The fundamental situation in particle physics these days is that we have an extremely successful theory, the SM, with only one sector of the theory untested. This is the Higgs sector, and it’s the source of a lot of the less satisfying aspects of the theory. The thing to be excited about is that later this year we should start getting experimental data relevant to this, seeing a Higgs if it is there. This could get very interesting, less so if the Higgs exists and behaves as predicted by the SM.

    Things like SUSY/strings/black holes/extra dimensions are a sideshow. These have never given any convincing explanation of anything about the SM or any serious reason to believe they exist. All that’s going to happen on that front is that over the next few years there will be a lot of null results coming from the LHC and other experiments. Sooner or later some of the partisans of these ideas will give up on them, other ones will keep the faith, but no one else will take them seriously. The collapse of some speculative research programs that have been around for decades may be of sociological interest to watch, but there’s not much of scientific interest there.

    Mike: if the SUSY breaking scale is much higher than the 100 GeV electroweak scale, SUSY does nothing at all to solve problems of the SM, all it does is introduce new ones.

  43. Paul Wells says:


    As you know there aren’t that many valid ways to build a point particle Quantum Field Theory- and supersymmetry is one of them. It may or may not exist but isn’t a search justified from a pure phenomenology point of view? I am thinking of the situation with Dark Energy a few years ago….

    Paul Wells

  44. Peter Woit says:


    I’ve nothing against people investigating supersymmetric QFTs. Actually, there is some fascinating mathematics behind the general idea of supersymmetry, and I wouldn’t be surprised if it somehow turns out to be of fundamental importance.

    The problem is that the supersymmetric extensions of the Standard Model that are known, once one includes supersymmetry breaking to agree with experiment, become quite complicated and don’t actually explain anything that the SM doesn’t. People like to go on about how SUSY relates fermions and bosons, but it doesn’t relate any known fermion to any known boson. It relates fermions to unknown bosons and bosons to unknown fermions. This isn’t convincing at all. But, sure, the LHC should look for it anyway, who knows what will be found?

  45. GradStudent says:

    The Cosmist,
    “From the outside it sounds like there is huge disagreement and confusion about basic questions, as if the whole field is in disarray like never before.”


  46. Pingback: Preliminary Results from XENON100? No WIMPs here! « Whiskey…Tango…Foxtrot?

  47. imho says:

    chuckle… me thinks HolyCow is a first year grad student 🙂

    These results and discussions are all signs of a healthy field, and as a cond mat guy I think this is all great. My question is where are the discussions about technicolor and its incipient resurgence???

  48. No Black Holes? says:


    “Things like … black holes … These have never …. any serious reason to believe they exist.”

    Am I understanding correctly that you’re saying there’s no serious reason to believe that black holes exist?

  49. milkshake says:

    Another layman question: How likely is that LHC will miss new physics? My understanding is that LHC detectors have to perform extensive data filtering, which in turn depends on how the triggers are set. So, my question is, would they be able to notice unexplained collisions against the high background, especially if signs of new physics were subtle ? Thanks

  50. SpearMarktheSecond says:

    Wimps don’t need SUSY, that dark matter has a weak interaction is sufficient. The existing ratio of dm to luminous matter is consistent with a weak interaction and thermal equilibrium in the big bang, but there are other solutions too. In any case mundane issues like hadronic uncertainties in the wimp-nucleon cross section are about an order of magnitude.

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